A hydrogen-containing gas is used as an anode gas in a fuel cell. A steam reforming method is generally employed as a formation method of the hydrogen-containing gas.
Specifically, for example, a hydrogen generating apparatus used in a fuel cell system has a reforming device that generates a hydrogen-containing gas from a raw material and water through a steam reforming reaction, and a heating device that feeds heat required for the steam reforming reaction to the reforming device. The reforming device is equipped with a reformation catalyst. The hydrogen generating apparatus is so constituted that a raw material of a hydrocarbon series, such as natural gas, LPG, naphtha, gasoline and kerosene, or an alcohol series, such as methanol, and water are supplied to the reforming device from an external supply infrastructure, the reforming device is heated with the heating device to a temperature suitable for the steam reformation reaction (reformation reaction temperature), and a hydrogen-containing gas thus generated is delivered from the reforming device.
In the steam reformation reaction, carbon monoxide (which is hereinafter referred to as CO) is formed as a by-product, and the hydrogen-containing gas delivered from the reforming device contains CO in an amount of approximately 10 to 15%. Remaining water is also contained as steam. CO contained in the hydrogen-containing gas poisons an electrode catalyst of a fuel cell to lower the electric power generation capability, and thus a hydrogen generating apparatus used in a fuel cell system is equipped with a unit that decreases the CO concentration in the hydrogen-containing gas.
As the method of decreasing the CO concentration, shift reaction is ordinarily used, in which CO and steam are reacted and converted to hydrogen and carbon dioxide.
Accordingly, the hydrogen generating apparatus has a converting device that performs shift reaction of CO and steam in the hydrogen-containing gas, and the converting device is equipped with a converting catalyst. The hydrogen generating apparatus is so constituted that the hydrogen-containing gas is supplied to the converting device at a temperature suitable for the shift reaction (shift reaction temperature), and the hydrogen-containing gas decreased in CO concentration is discharged from the converting device. In many cases, the CO concentration in the hydrogen-containing gas is decreased to approximately 0.5% or less by the converting device.
For further decreasing CO, a CO removing device equipped with a selective oxidation catalyst is provided on the downstream side of the converting device, and CO in the hydrogen-containing gas is oxidized by supplying air to the CO removing device, thereby decreasing the CO concentration in the hydrogen-containing gas to 100 ppm, and preferably to 10 ppm or less. The hydrogen-containing gas containing hydrogen discharged from the CO removing device as a major component is supplied to a fuel cell for electric power generation.
As the converting catalyst, such a catalyst is used as a noble metal catalyst, e.g., platinum, ruthenium and rhodium, a Cu—Zn catalyst and a Fe—Cr catalyst.
Upon using a Cu—Zn catalyst as the converting catalyst among these catalysts, the shift reaction proceeds in a reducing atmosphere. Accordingly, for preventing the catalyst capability from being deteriorated, it is desirable to suppress the catalyst from being oxidized, thereby maintaining the reducing atmosphere even in the shutdown state of the hydrogen generating apparatus.
When steam remains inside the converting device in the shutdown state of the hydrogen generating apparatus, there are cases where the converting catalyst absorbs water produced by condensing the steam through a decrease in temperature, and the absorbed water is evaporated through an increase in temperature upon the next start-up, thereby disrupting the converting catalyst.
Accordingly, such a method is proposed that in the shutdown operation of the hydrogen generating apparatus, the temperature of the reformation catalyst is decreased while supplying the raw material and water, and when the temperature of the reformation catalyst reaches the prescribed temperature, at which thermal decomposition of the raw material occurs, (for example, 300° C. for butane gas) or lower, supply of water is stopped, and the converting device is purged with the raw material (see, for example, JP-A-2000-95504). According to the aforementioned shutdown operation, steam can be purged from the converting device as much as possible, and the catalyst can be suppressed from being oxidized.
Such a method is proposed that in the shutdown operation of the hydrogen generating apparatus, purge with steam is performed, and then after the temperature of the reformation catalyst layer is decreased to a temperature that is equal to or lower than the temperature, at which thermal decomposition of the raw material does not occur, and is equal to or higher than the condensation temperature of steam, the raw material is supplied to the converting device (see, for example, JP-A-2002-151124).
In the hydrogen generating apparatus of JP-A-2002-151124, the reforming device is purged with the raw material, and the raw material discharged from the reforming device is supplied to the converting device, thereby purging the converting device with the raw material. On the other hand, JP-A-2000-95504 discloses such a constitution that the raw material is supplied directly to the converting device from the raw material supplying device, thereby purging the converting device with the raw material independently from the purge of the reforming device with the raw material (see, for example, FIGS. 2 and 3 of JP-A-2000-95504).
The entire disclosure of JP-A-2000-95504 and JP-A-2002-151124 are incorporated herein by reference in its entirety.